Optical detection via defect-enhanced carrier generation in SOI ridge waveguides is now established as a viable method for sub-bandgap optical to electrical conversion. (See: A. P. Knights et al. “Silicon-on-insulator waveguide photodetector with self-ion-implantation engineered-enhanced infrared response.” J. Vac. Sci. Technol. A24, 783-786 (2006); M. W. Geis et al. “CMOS-compatible all-Si high-speed waveguide photodiodes with high responsivity in near-infrared communication band.” IEEE Photonics Tech. Lett. 19, 152-154 (2007); and Y. Liu et al. “In-line channel power monitor based on Helium ion implantation in silicon-on-insulator waveguides.” IEEE Photonics Tech. Lett. 18, 1882-1884 (2006).) In the previously reported work, defects are introduced into the waveguide through ion implantation (with or without a post-implantation thermal anneal), which increases the optical absorption for wavelengths around 1550 nm through the (essentially) mid-gap divacancy or interstitial cluster level (see, H. Y. Fan and A. K. Ramdas “Infrared absorption and photoconductivity in irradiated silicon.” J. Appl. Phys. 30, 1127-1134 (1959)). Integrated p-i-n diode structures are used to extract the optically generated carriers from the device volume supporting the optical mode, thus allowing for signal detection directly from the waveguide. The degree of absorption may be changed by varying the concentration of defects, and thus the amount of signal that is sampled may be varied from a few percent to virtually the entire signal. As a result, defect-enhanced photodetectors may be implemented as both in-line power monitors and as end-of-line signal detectors. Their potential advantages over competing technologies rely on the fact that they are fabricated entirely using standard silicon processing methods and do not involve hybrid integration or the hetero-growth of germanium.
The photodetectors reported to date are fabricated in the intrinsic (or low-doped) silicon overlayer of a silicon-on-insulator (SOI) structure, and therefore the influence of background dopant concentration on device performance has not been studied. In the case of carrier generation via the divacancy defect, the background dopant concentration will affect the charge state of the divacancies, which in turn will influence the defect mediated absorption. Evidence consistent with this postulate has been reported previously (see, C. S. Chen & J. C. Corelli “Infrared spectroscopy of divacancy-associated radiation-induced absorption bands in silicon.” Phys. Rev. B 5, 1505-17 (1972)), and has recently been demonstrated using a waveguide geometry (see, D. Logan et al. “The effect of doping type and concentration on optical absorption via implantation induced defects in silicon-on-insulator waveguides.” in COMMAD 2008 IEEE Proc. Conf. on Optoelectronic and Microelectronic Materials and Devices. (Sydney, Australia, 2008). pp. 152-5).
The divacancy has a deep-level situated in the band gap at Ec-0.4 eV and as such, light at a wavelength of 1550 nm may cause charge excitation from the valence band or from the deep-level to the conduction band, albeit at significantly different rates (see, E. Simoen et al. “Impact of the divacancy on the generation-recombination properties of 10 MeV proton irradiated Float-Zone silicon diodes.” Nucl. Instruments and Methods in Physics Research A. 439, 310-318 (2000)). The variation in cross-section for these two processes results in a measurable difference in absorption coefficient as background doping type and concentration is changed, but there is no doping concentration at which the absorption coefficient related to the defect is reduced to zero (see, Logan, supra.).